Communications technologies and uses have greatly changed over the last few decades. In the fairly recent past, copper wire technologies were the primary mechanism used for transmitting voice communications over long distances. As computers were introduced the desire to exchange data between remote sites became desirable for many purposes such as those of businesses, individual users and educational institutions. The introduction of cable television provided additional options for increasing communications and data delivery from businesses to the public. As technology continued to move forward, digital subscriber line (DSL) transmission equipment was introduced which allowed for faster data transmissions over the existing copper phone wire infrastructure. Additionally, two way exchanges of information over the cable infrastructure became available to businesses and the public. These advances have promoted growth in service options available for use, which in turn increases the need to continue to improve the available bandwidth for delivering these services, particularly as the quality of video and overall amount of content available for delivery increases.
One promising technology that has been introduced is the use of optical fibers for telecommunication purposes. Optical fiber network standards, such as synchronous optical networks (SONET) and the synchronous digital hierarchy (SDH) over optical transport (OTN), have been in existence since the 1980s and allow for the possibility to use the high capacity and low attenuation of optical fibers for long haul transport of aggregated network traffic. These standards have been improved upon and today, using OC-768/STM-256 (versions of the SONET and SDH standards respectively), a line rate of 40 gigabits/second is achievable using dense wave division multiplexing (DWDM) on standard optical fibers.
In the access domain, information regarding optical networking can be found in Ethernet in the First Mile (EFM) standards (IEEE 802.3ah which can be found at www.ieee802. org and is included herein by reference) supporting data transport over point-to-point (p2p) and point-to-multipoint (p2mp) optical fiber based access network structures. Additionally the International Telecommunications Union (ITU) has standards for p2mp relating to the use of optical access networking. Networks of particular interest for this specification are passive optical networks (PONs). For example, three PONs of interest are, e.g., Ethernet PONs (EPONs), broadband PONs (BPONs) and gigabit capable PONs (GPONs), which are displayed below for comparison in Table 1.
TABLE 1Major PON Technologies and PropertiesCharacteristicsEPONBPONGPONStandardIEEE 802.3ahITU-T G.983ITU-T G.984ProtocolEthernetATMEthernetRates (Mbps)1244 up/1244 down622/1244 down1244/2488 down155/622 up155 to 2488 upSpan (Km)102020Number163264of Splits
An exemplary GPON 100 in FIG. 1 shows elements of an optical distribution network (ODN) that interact with various endpoints of an optical network termination (ONT). Additionally GPON 100 uses wave division multiplexing (WDM) on the optical signals. As shown in FIG. 1, one or more service providers or types 102 can be in communication with an optical line termination (OLT) 104, which is typically located in a central office (CO) (not shown). The OLT 104 provides the network side interface and is typically in communication with at least one optical network termination (ONT) (or an optical network unit (ONU) which performs similar duties as an ONT but typically for a multi-dwelling unit). These service providers 102 can provide a variety of services such as video-on-demand or high definition television (HDTV), Voice over IP (VoIP) and high speed internet access (HSIA). The OLT 104 transmits information to WDM 106 which multiplexes the data and transmits the data optically to a passive combiner/splitter 108. The passive combiner/splitter 108 then splits the signal and transmits it to the upstream WDMs 110 and 116. These WDMs 110 and 116 demultiplex the signal and forward it on to their respective ONTs 112 and 118. These WDMs (108, 110 and 116) are typically integrated into both the OLT and the ONTs and are used for placing and extracting the upstream and downstream wavelengths depending upon their locations in the optical network. These ONTs 112 and 118 then forward the information onto their respective end users (EU) 114, 120 and 122.
It will be understood by those skilled in the art that this purely illustrative GPON 100 can be implemented in various ways, e.g., with modifications where different functions are combined or performed in a different manner. For example these WDMs (108, 110 and 116) typically are duplexers, but if an additional signal is being transmitted, e.g., a cable-television signal in a GPON, they can act as triplexers. Additionally in the upstream direction, the optical signal would typically have a different wavelength from the downstream signal and use the same WDMs 106, 110 and 116, which have bidirectional capabilities.
With the advent of the above described services and the ongoing improvements in optical networks, many telecommunication companies are choosing to upgrade their copper centric access networks with fiber optic access networks. Some such upgrades include, for example, using one of the above described PON networks combined with fiber to the home (FTTH), and/or hybrid networks, e.g., fiber to the cabinet (FTTC) combining optical EFM and/or PON for data backhaul with very high speed digital subscriber line (VDSL2) by reusing the last hundred meters or so of copper wire. These upgrades allow an increase in the types and quality of services delivered by companies to end users. A comparison of two different types of optical distributions networks (ODNs) are summarized below in Table 2.
TABLE 2P2P vs. P2PMP2PP2PM (GPON)Mature technology, low riskNew technology, higher riskFavored by non-telcosFavored by T1 (closed network)in open networkMain markets: Northern andMain markets: US and Southern EuropeWestern EuropeLowest CapEx todayLow OpEx today, higher price erosion
Regardless of which type of optical system, i.e., p2p or GPON (or both), is deployed, one of the primary requirements for low capital expenditure (CapEx) and operational expenditure (OpEx) is for the optical system to employ a passive ODN, e.g., using only passive optical components between the central office (CO) and the user equipment (FTTH) or the cabinet (FTTC). Examples of the passive optical components include connectors, fibers, splices and passive power splitters (PPS). A downside to using only passive optical components is that the overall signal reach becomes reduced as a function of the number of splits in the system. For example, in a typical PON which is communicating with up to 64 end users, the effective usable signal strength distance is approximately 20 kilometers.
An acceptable amount of loss allowable attributable splitters (also referred to as “splitter insertion loss”) in an ODN is specified by, for example, the G.984.2 specification for GPONs depending upon optical class. For more information regarding GPONs in general, the interested reader is directed to the G.984.1-4 standards which can be found at www.itu.int/rec/T-REC-G/en, the disclosure of which is incorporated herein by reference. Three general optical classes are class A optics (which allow for a loss between 5 to 20 dB), class B optics (which allow for a loss between 10 to 25 dB) and class C optics (which allow for a loss between 15 to 30 dB). A current industry standard used for GPONs is considered to be a B+ optics class which allows for a maximum loss of 28 dB over an ODN. In other words, the optical transceivers in an OLT and the ONT(s) should be able to perform to provide an acceptable output on an ODN where the passive components, e.g., splices, connectors, fibers and splitters, together have an insertion loss of 28 dB. This link budget also typically needs to take into account other power penalties and some amount of system margin.
Different passive components within an ODN provide different amounts of loss during transmission. Table 3 below shows typical ODN components and the associated loss.
TABLE 3Typical ODN Components and Associated LossesComponentAverage LossDescriptionSingle Mode Fiber0.4 dB/km @ 1310 nmG.652.B0.25 dB/km @ 1550 nmConnector/Splice0.1-0.2 dBLC/PC TypePassive1 × 4 7.5 dBStandard Grade Powersplitter/Combiner1 × 8  11 dBSplitter1 × 1614.2 dB1 × 3217.8 dB1 × 6421.1 dB1 × 12823.8 dBAs can be seen in Table 3, the splitter typically contributes the largest amount of loss in an ODN. For example, the loss associated with a splitting ratio of 1:64 (which is a commonly desired ratio today) is 21.1 dB, which roughly equates to the loss generated by passing an optical signal through a fiber with a length of 53 km (e.g., the fiber loss at the 1310 nm wavelength for transmissions over 53 km of single mode fiber). The fiber loss over that distance is shown in the equation below.53 km×0.4 dB/km=21.2 dB   (1)Looking at the optical losses due to the splitters from another perspective using the data in Table 3 above, for a B+ system with a 28 dB link budget, a 1×64 splitter would reduce the reach (i.e., the useful transmit distance) of the PON to 18 km. Thus, even small reductions in the splitter insertion loss could result in appreciable increases in PON optical signal reach or possibly the number of splits while maintaining a similar reach. For example, a doubling of the split ratio implies a +3 dB increase in insertion loss which equates to approximately 7.5 km in reach.
Other link budget considerations also exist and should be addressed to extend the reach of a PON. One issue is that for EPONs and GPONs the upstream transmission structure typically has a shorter reach as compared to the reach in the downstream transmission direction. The cause for this difference is inherent to the time division multiplexing/time division multiple access (TDM/TDMA) protocol structure used on the PONS, as will be described in more detail below. The division between downlink and uplink is done via WDM where the downlink operates on a wavelength of 1490 nm with a bandwidth of 20 nm and the uplink operates on a wavelength of 1310 nm with a bandwidth of 100 nm. The data in the downlink is broadcasted to all ONTs in the PON using a TDM scheme where each of the ONTs takes data from its assigned timeslot in the downstream signal. The downstream optical signal is a continuous wave with equal power transmitted towards all ONTs. The optical terminal transceiver (OTRx) located in the OLT is shared by all ONTs and thus can contain high quality optics with a high output power.
In the upstream direction, a TDMA scheme (e.g., as shown in FIG. 2) is used where ONTs 202 and 206 are allowed to transmit data in granted time-slots on their optical wavelength(s). This means that ONTs 202, 206 transmit in a burst mode at their allotted time slots, as compared to a continuous power transmission in the downstream direction from the OLT 210. Since the ONTs 202, 206 are located at different distances from the OLT 210, the ONTs 202, 206 are informed by the OLT 210 when, and with what power, to transmit their respective bursts so that the ONTs signals are arriving in an aligned time structure at the OLT 210. For example, ONT1 202 receives the continuous transmission 212 and receives its information from its assigned time slot 204. ONT2 206 performs similar functions and receives its information from timeslot 208. Based on the received data the ONTs know their transmission time slot which results in an upstream message 214 where the different ONT outputs are in a time sequential order.
Given this TDMA approach, the OLT 210 typically includes a burst receiver that decodes the ONTs data which arrives slightly jittered (or asynchronous) with differing power levels. At higher data rates of transmission this decoding process becomes more challenging to perform. For example, currently systems operating at transmission rates of 1.25 Gbit/s are considered to be cost efficient, transmission rates on the order of 2.5 Gbit/s are considered to be technically feasible, while transmission rates of 5-10 Gbit/s are not currently considered feasible in this type of optical communication system. This leads GPONs to have an asymmetric data rate. The use of a burst receiver introduces a burst penalty in the area of 3-6 dB depending upon the quality of the components in the OTRx. Coupling this burst penalty with a slightly higher loss on the upstream band (approximately 0.15 dB/km) and the need to use less expensive optical components (diplexer, triplexer) in the ONTs due to scalability reasons, transmission in the upstream direction becomes the limiting direction for this type of optical system.
To overcome this challenge of obtaining a greater transmit distance with a usable optical signal, while also maintaining a high number of allowable splits, different possible solutions can be considered. Generally, either the losses introduced by the splitters need to be reduced, the signal needs to be amplified or both. Regarding the possible solution of amplification, a variety of options exist as illustrated generally in FIG. 3. Therein, three potential locations for adding a booster for amplification are the CO 302, a remote node 304, or with each ONT 310 and 312 at a location such as home1 306 and home2 308. The booster in the CO 302 is shown as booster 314 near the OLT 316, the booster in the remote node 304 is shown as booster 318 downstream of the passive combiner splitter 320 and in the homes (or near the ONTs) the boosters are shown as boosters 322. Putting a booster or amplifier in any of these locations brings with it different, associated problems. For example, it would be cost prohibitive for a booster 322 to be located with each ONT 310 and 312 due to the high numbers of ONTs in a system. If a booster 318 were to be put in a remote node 304, it would add a need for power and perhaps more maintenance visits, to an otherwise passive location. Regarding placing a booster 314 in the CO 302 near the OLT 316, this also is not unproblematic since the booster 314 can only be operated in a low power mode due to non-linearities on the fiber. Moreover, since a typical GPON system is upstream limited and the input sensitivity of a pre-amplifier in the OLT booster 314 is approximately −28 dBm, this solution would be an added expense with no value for the upstream signal.
Accordingly the exemplary embodiments described herein provide systems and methods that allow, e.g., for either reducing signal loss or improving the optical signal strength in a PON.